This invention relates generally to power conversion systems. More particularly, this invention relates to LLC resonant converter circuits that include multiple transformers for providing substantially matched currents to multiple output loads.
In some electronic circuit applications, to reduce size and weight and to minimize cost, a single power supply may be used to supply matched currents to multiple load circuits. For example, in a light-emitting diode (“LED”) television, a single LLC resonant converter may be used to drive multiple LED strings, where each LED string includes multiple series-connected LEDs, with the same current supplied to each LED string.
One such previously known LLC resonant converter circuit, referred to as multi-transformer LLC resonant converter 10, is illustrated in FIG. 1. Multi-transformer LLC resonant converter 10 includes inverter 14, resonant capacitor 15, transformers 161 and 162, and rectifier/filter circuits 181 and 182, respectively, and provides substantially equal output currents I1 and I2 to LED strings 121 and 122, respectively. Transformer 161 includes leakage inductance Ls1 and magnetizing inductance Lp1, and transformer 162 includes leakage inductance Ls2 and magnetizing inductance Lp2. Primary windings P1 and P2 of transformers 161 and 162, respectively, are coupled together in series, and the series-coupled primary windings are coupled to inverter 14 via resonant capacitor 15.
For high efficiency, LLC resonant converters are typically operated using primary-side zero voltage switching (“ZVS”), which requires large magnetizing currents I1p1 and I1p2. Indeed, I1p1 and I1p2 may be a large fraction of primary currents Ip1 and Ip2. To supply matched output currents I1 and I2 to LED strings 121 and 122, primary currents Ip1 and Ip2 must be matched. As a result, to provide substantially equal primary currents Ip1 and Ip2, magnetizing currents I1p1 and I1p2 must be substantially equal, which requires that the tolerance of magnetizing inductances Lp1 and Lp2 must be impractically small.
One previously known LLC resonant converter circuit, referred to as LLC resonant converter 10′, that attempts to solve this problem is illustrated in FIG. 2. In particular, LLC resonant converter 10′ includes an additional transformer 163 between inverter 14 and resonant capacitor 15 and transformers 161′ and 162′. In this circuit, a single magnetizing inductance Lp3 of transformer 163 provides the necessary shunt inductance of the LLC resonant converter. As a result, the magnetizing inductances of transformers 161′ and 162′ can be made very large, which renders the effect of any magnetizing inductance tolerances insignificant.
However, the circuit of FIG. 2 has several significant disadvantages. First, the power from the DC input Vin to each output must be processed through two transformer stages, which degrades efficiency. In addition, because transformer 163 must be sized for the full output power, the size and material cost of transformer 163 are substantial.
Accordingly, improved LLC resonant converter circuits for driving multiple loads with substantially matched output currents are desirable.